The config file is expected to be named config.json and located in the
current working directory. If it's not, you can pass in a relative path.

$ telepad some/other/file.json

The special buttons on the right of the Launchpad will light up red for each
phone number in your config. Press them to make a call, you'll see them turn
green. Each row corresponds to a phone and each button corresponds to a
sound from your config!

With thanks to Syd Lawrence for midi-launchpad and his helpful advice.